Process and apparatus



17, 1936. w s ET AL 2,34fi

PROCESS AND APPARATUS Filed Oct. 3l, 1952 3 Sheets-Sheet 1 miuiam icalcou INVENTORJ- (AnuwonLl PBenfiinti Q TTORNEY W. S. CALQOTT ET AL.

PROCESS AND APP RATUS arch 17 13.,

Filed Oct. 51, 1932 3'Sheets-Sheet 2 minim icalwn ATORNEY.

1935' w. s. CALCOTT ET AL PROCESS AND APPARATUS Filed m. 51, 19:52 7 5 sheets-sheet 3 llll l l l nnfig'ml Pennini alum-sewn MW Patented Mar. 1 7, 1936 PROCESS APPARATUS William s: Calcott and Anthony F. "Benning,

Pennsgrove, N. J assignors to E. I. du Pont de Nemours & companm'wilmington, Del., a cornotation of Delaware Application October 31, 1932, Serial No. 640,554

2': Claims. (01. 204-9) This invention relates to fluorine, and a process for the production thereof. It further relates toan apparatus suitable for the production of fluorine. 4

It is an object of the invention to provide a new and improved type of apparatus suitable for the production of fluorine. A further object is the provision of a new and improved process for the production of fluorine. Other objects are the Provision of a practical'fluorine cell, the construction of an apparatus for the electrolysis of substantially anhydrous hydrofluoric acid-the.

construction of an apparatus in which anhydrous hydrofluoric acid can be electrolyzed by an intermittent and/or continuous process, and the construction of new and useful indicating and safety devices for a fluorine cell. Other objects will appear hereinafter.

These objects are accomplished by the electrolysis of substantially anhydrous hydrofluoric acid in an electrolytic cell in which the electrodes are not substantially attacked by fluorine and/ or hydrofluoric acid.

The character of-the apparatus falling within the invention is subject to considerable variation and modification in the manner of its practical construction but may be illustrated by the various forms of apparatus shown in the accompanying drawings, in which similar numbers refer to similar parts throughout the several views and the description thereof.

In the drawings: Fig. 1 is a side view partly in section and somewhat diagrammatic of one form of the apparatus for carrying out the invention.

Fig. 2 is a view partly in section of a modified firm of a portion of the apparatus shown in g. 1. Fig. 3 is a perspective viewpartly in section of 49 certain portions of the apparatus shown in Fig. 1.

Fig. 4 is a modified form of a portion of the indicating apparatus shown in Fig. 1.

Fig. 5 is another modification of the indicating apparatus shown, in Fig. l. 4' Fig. 6 is a view partly in section of a portion of the indicating apparatus shown in Fig. 1.

Fig.7 is a modification of a portion of the indicating apparatus shown in Figs. 1 and 6.

Fig. 8 is a view partly in section showing a modifled arrangement of a portion of the indicating and electrolyzing apparatus illustrated in Fig. 1. Fig. 9 is a perspective view showing means for the circulation of the liquids within the apparatus,

illustratedinFig. 1. Fig. 10 is a sectional view of a portion of the apparatus illustrated in Fig.2 showing a modified means for maintaining one of the electrodes in position.

Fig. 11 is another sectional view showing a still further modified arrangement for maintaining 5 one of the electrodes, illustrated in Fig. 2, in position. 4 Referring to Fig. 1 of the drawings, the apparatu's illustrated comprises a container 2 in which the electrolyte is held. A'removable top or cover 10 d is attached to the outer casing of container 2 by a suitable fastening means, such as indicated at 6. The casing 2 is surrounded by a jacket 8 having an inlet I0 and an outlet l2 which permit the circulation of a heating or cooling fluiddn 15 heat exchange relationship for the purpose of maintaining the desired temperature within the 5 cell. The cover 4 has a depending annular partition consisting of a solid metal wall l3 from. which there depends a diaphragm in theform of go screen It. This annular partition divides the cell into two compartments. Within the inner compartment and attached to the top by suitable means is an anode IS. The anode compartment has an opening at l8 for the, entry of electrolyte g5 material, an opening 20, for the escape of liberated fluorine, and an opening 22 which is connected with the electrolyte levelindicating device herein described. Within the outer compartment and attached to the top by suitable insulat- 80 ing means is a cathode 24 shown in the drawings as a screen attached to vertical rods, such as indicated at 26. These rods are'held in position by suitable stuiflng box devices indicated at 28. A packing substance 30 within the stufilng box 36 device serves to insulate the cathode 24 from. the cover 4. The. cathode compartment has openings 32 and 34 for the admission of the electrolyte substance and an opening 36 for the escape of liberated hydrogen. 40 In normal operation, the cell is preferably filledwith' the electroylte to about the level indicated in Fig. 1, although the liquid level may vary widely. An electrolyzing current is supplied to the cell walls and top at a convenient terminal 45 suchas shown at 38- to cathode 24. by a suitable connection 40. Flourine is liberated-at the anode and escapes through opening 20 into a connect ing line 42. This line is branched as shown at 44, one branch connecting with a U shaped msnometer tube 46, containing a suitable fluid l8 suchas, for example, anhydrous hydroflourlc acid saturated with a soluble fluoride. The other branch is connected to a trap 50. The fluorine leaving the anode compartment 5| after through line 52 into coil 54 to a suitable storage container (not shown). 001164 is s'urrounded by a cooling substance (not shown) held any substanc'eliquefying 2 opening 20 and line 42 mm trap so and therein container 56." Coil 54 is preferably sufliciently large in internal diameter so that flourine may pass through and iatthe same time may allow in the coil, for example, drofluoric acid=to1return to the-trap:50. Irapv 50 has; a bottom- 'outle t -58 "branching into two 60 and 62. It it'is desired to retflrnith" through opening 36 and line 60 into line "I0, which is connected at one end with. manometer 46 and at the other with trap I2. From line I0, the

- vapors pass into trap "I2 and then into coil I4 ing at the top 86 connected to the opening 22 at 1 34 so that the level of theelectrolyteinwell 64 is which is surrounded by a suitable coiling medium (not shown) held in container I6. The temperature of coil 14, as well as that of coil 54 is preferably such as to condense vapors of hydrofluoric acid. Coil I4, as in the ca'seot coil 54, is preferably sufliciently large to allow gases to pass upward and the condensate to flow downward'at the same time. The condensate collected in trap 12' may be returned to the cathode compartment through the bottom outlet I5, line 11, valve 18, line 68 and opening 36 (valve 80 being closed) Ii de-' sired, all or a part of the condensate from trap I2 may be returned to the compartment insteadoi' the top by closing valve I8 and opening valve 80, thereby causing-thecondensate to pass through line 82 and enter the cathode compartment through opening 32.

A liquid level indicating device is provided to indicate the level of the electrolyte in the cell. This device consistsof a well 84 having an openthe top of the anode compartment by means of a line 88 and also having an opening at the bottom through line 90 which connects with line-92, en-

tering the cathode compartment through opening the same as the electrolyte levelin the-container 2; Extending different depths are separately connected to terminals 93. The

wires generally indicated at 94 are insulated ina suitable manner from the well current source (notshown) is ester 91 through line 98 andalso; tothe wall' oi .well' 84 at the top as. 1A

selector switch as. in ,contact with the. terminals The cell operation fi03 until a current through the electrolyte in ,well 64 is shown on indicatort'ls' f tinuous, as-desired. The design of. the apparatus may be varied'considerably; .It may be desirable, for instance, 'to' force: the condensate from-'condensers; 54 and.'|4j'through lines 62 and 82,.re- I spectively, by means'ot pumps (not shown) connected in said lines. It desired,'the electrolyte may be introduced into the cell or replenished, for

w example, by .the addition orhydroiiuoricacid.

through valved inlets 33.60am 6 2.

it. "In-Fig.2, the container I02- with a cover I04 held in place bya suitable fas- :havinganinlet H0 and an outlet .desired temperature withinvthe cover I04 has a depending annular partition conbottom of' the cathode- "1 and extendsto a liquid (not shown). and a. reservoir of the electrolyte (not shown) as inF igpl.

.-by conveniently placed terminals, as intogwell 84 are a series of wires generally indicated-atasl which connected to indi-.

through a wire '99." The depth of the/1 electrolyte "is. then determined bymoving the"; illustrated 1n.Fig.,-, 1 are may jbeuintermittent or conthe cathode compartment in the central portion of ,the cell with the anode compartment surroundtening'means, such-as indicated at I06. As in Fig.1, the container is surrounded'by a Jacket I08 II2 which permit the circulation of a' fluid in r"elation'ship forthe purpose of maintaining the cell. The top or sisting of swell H3 having a depending screen II4, dividing the cell into two compartments. 15

Within the inner compartment and attached to the top by a. suitable means-such as indicated at H5 is a cathode I24 shown in the form. of an annular screen or grid extending nearly to the bottom ofthe cell. the cover by means of a suitable insulating medium held in a stufling box I28. The cathode compartment hasan opening I23 connected to a liquid level indicating device (not shown) such as described with reference to Fig. 1, and a second 25 opening I36 for the escape of the gases formed during operation of the cell. This second opening I36 is connected to a condenser (not shown) such as described with reference to Fig. 1, and if desired, the. condensate from said condenser may 30 be allowed to run into the cathode compartment through this opening. Alternatively,as indicated in the description of Fig. 1, the condensate may be returned to the cathode compartment at the bottom through line I32 and opening I34.

Within'the outer compartment and attached to the top I04 is an-anode II6 attached to vertical rods I26. The anode compartment has an opening at-the top through line I20 which serves as an outlet for fluorine liberatedv at the anode. 40

Line I20 may. be connected to a suitable trap and condenser (notxshown) such as described. with reiferenoe to Fig. 1, and if desired, the condensate may be returned. to the anode compartment. through this line. Alternativelm the'con- 45 densate may'be introduced into the bottom of. the anode compartment through line I62 and opening H8. Line I92 is similar to line 92 in Fig. level indicating device paratus of-Fig.'2,-which are not shown, may be similarto those describedin Fig. 1. The operation of the cells described in Figs. 1 and 2 may be substantially the same as regards the kind and amount of electrolyte,- the current density, and the temperatures and pressures of operation.

In Fig.. 3, certain portions of the apparatus are the container 2, the cathode 24, and the to the device shown in '4, More extended different indicating de light bulbs.

ces :291; as for example-electric is provided 5 heat exchange 10 The cathode is insulated from 20 shownjin perspective.

Wires toa source of current..are 7' shown at m and 269,'said wires'belng conmcted to the selector switch 293'and the well as, re-

"spectively; For convenience of illustration, portions of the well 284 and the wires 294 are broken away. The .wires'294 are insulated from well 296 by any suitable means, e. g., as hereinafter described with reference to Figs. 6 and 7. The operation of this .liquid level indicating device is substantially the same as that shown in Fig. 1. In the modification of the electrolyte level indicatingdevice shown in Fig. 5, a solenoid 499 is connected atone end to the electrolyte, well 384 (shown with parts broken away) and at the other to a source of current by means oi a wire .399.

The other wire from the source of current is con-'- 'nected to a selector switch 396 by means of a wire 398. Selector switch 396 may be moved in order to contact the terminals 393 of the wires generally indicated at 394,- which extend 'difierent depths into the electrolyte well 394. Wires 994 are insulated from well 384 by any convenient method. When a current passes through .the solenoid, the piston 39lis drawn inward closing a battery circuit 389 connected to an indicator Figure 6 illustrates a method of insulating the wires 494 which extend to diflerent depths into the electrolyte well 484. This insulation is effected in any suitable manner, e. g., by means of plugs consisting of a mixture of powdered aluminum fluoride and a wax such as ceresin generally indicated at 495. A modiflcationof the method of insulation is shown in Fig. '7. wherein the wire indicated at 594 leading into the electrolyte well 594 passes through a packing box 695 containing the insulating composition. The remaining wires (not shown) may be similarly insulated.

If desired, the liquid-level indicating device may be included directly in the cellcontainer. This is illustrated, for example, in Fig. 8 which shows an upper corner of the cell partly in section and withparts broken away. In Fig. 8, the'cell container is indicated at 692 and the top 699 is suit ably fastened, for example, by means of bolts such as: shown at 699. A depending. wing 695 forms an electrolyte well with the casing 602. Thewires 694 connected to a suitable indicatin device, e. g;, such as described in Fi s. 1. 4 and 5.

. extend different depths into this electrolyte well 7 through an insulator 996.

It may be desirable to to prevent looalconcentration. This may be ef.- fected by suitable mechanical means or by circulation of the electrolyte, e. g.'.'asgillustratedin h Fig. 9. InFig.-9 a line 121 is] shown leading from. the bottom of the celll92 to a pump I29 which tin-n connects to a point higher in the cell through a line 131. The electrolyte may thus be circulated intermittently or continuously.

In Fig. 10, an upper corner of the cell is shown partly in section and with parts broken away,-

illustratingm modification of the apparatus describedinFigzxThe portionsshownare apart of thecasing 992 and apart of thetop 994 which is held-in position by a fastening device such as indicatedat 996. A part of the partition wall "3 isalso-shown with-parts broken away. Ac-

cording to thiamodiflcation, the anode, shown wltliparts' broken away at "6, is welded to a projecting fastening device Ill which in turn is welded to the casing 992.

Another modification of the method of fastening the anode within the cell of-Flg. 2 is shown in Fig. 11 in which the'casing i's'indieated at 992,-

the top at 99l,'the partition at M3 and the anode at 9", all'with parts broken away- The-top is.

agitate. the electrolyte held. in position by a fastening means such as shown at 996. In this mpdiflcation, an L shaped place such as shown at 9" ,is i-fastened to the anode and another L shaped piece9l9 in inverted position is fastened to the casing 902., 'By allowing the L piece'9l'l to rest onthe inverted L 9|9, the anode maybe held in position and at the very highly desirable results. In practice,'we have found that a cell having a substantially pure nickel anode may be operated continuously or intermittently over a longperiod of time...

Nickel alloys, preferably containing'materials other than nickel in relatively small amounts,

- hydrofluoric acidand' fluorine. Nickel has given may be used as anode materials. Metals'whieh' may be alloyed with nickel for this purpose are, for example, iron, chromium, and/or cobalt. The

amount of nickel which should be present in a nickel alloy to give desirable results may be determined by empirical, tests. In general, the

" proportion of nickel should be sufliciently high to render the alloy non-polarizing under the conditions of reaction. The use of an anodecontaining nickel in a cell of this typeis apparently novel. It should be noted that many materials of construction such as platinum or graphite which are useful as anode materials in many types of electrolytic cells are practically 'worthless for 'the commercial electrolysis of anhydrous hydrofluto the action of hydrofluoricacid and fluorine.

to the action of anhydrous ihydro'fluoric acid .and hydrogen. As specific examples of materials'of construction which maybe employed. may be mentioned: iron (including; greyjand white cast .iron) nickel, cobalt, copper,-'chromi ui n, magnesium and alloys of any twoor-more ofthese meturic acid since they are notsufliciently resistant als. .In operating the cell with a nickelanode and an iron cathode, it has beninoted'that the nickel may plate out on :the iron at the begin ning oithe operation.= If desired, a nickel plated an'iron alloy.

non-anodic material resistant to;the"action of anhydrous hydrofluoric acid,.fluorineand hydrogen. As indicated by the foregoingdescription I with reference tothe: drawings, this partition abovethe level of the electrolyte should prefer-.-

occur' with explosive violence CBelow thelevel' in the form ofa pervious" diaphragmyas tor example, a screen or'grid "having suiilcientlysmall apertures to prevent thepassageiofj large bubbles of fluorine. The whole partition is preferably.

The partition separating anode :and* cath- I ode compartments may I be constructed of any.

I cathode may be =used initially ll ,In'jenerhl, itis preferable to use a cathode construot'ed of iron or of the-electrolyte, the partition between the anode and cathode compartments. shouldpreferablybe constructed of a metal resistant'tothe action of hydrofluoric acid, fluorinesand. hydrogen. That part oi. the partition-above the surface .of-the electrolyte may or maynot-"beof'the same mate-.

' results have been obtained in the use of partianode and cathode compartments. The anode,

tions constructed of iron and steel. Where a cast iron has been used slightly better results have been obtained with white cast iron than with grey cast iron.

The container or casing of the cell and the cover may likewise be constructed of any nonanodic materials resistant to the action of anhydrous hydrofluoric acid, fluorine and hydrogen. Very highly desirable results have been obtained in the use of metals for this purpose. As specific examples of metals which may be employed may be'mentioned iron, steel, cobalt, chromium, magnesium,.copper and alloys of any two or more of these metals, or anodically passive alloys of any one or more of these metals with nickel. Par ticularly desirable results have been obtained in the use of iron, steel and alloys thereof. In operation, the container may be electrically insulated from-the anode or from the cathode or from both. For convenience of construction, 'as well as from the standpointof corrosion, however, it has been found that especially desirable results are obtained in the construction of a cell consisting of a metal container, an anode, a cathode and a metal partition between the anode and cathode compartments of the character described, in which the anode and the metal partition are always electrically connected to the metal casing of the container and the cathode is always electrically insulated, Certain of the specific metals which have been described as being useful in the construction of the metal container and the partition between the anode and cathode compartments are characterized by the property that they become passive or polarize when an electrolyzing current is passed through them in contact with anhydrous hydrofluoric acid. Hence, by having the metal container and the partition connected to the anode and constructed of metals of this character, these portions of the apparatusare always anodically passive. The use of anodically passive metals apparently tends to minimize corrosion. Insofar as' isknown, this type of construction is novel in an electrolytic cell.

The size and shape of. the various portions of this cell, including the casing or container, the anode, the cathode and the partition wall between the anode and the cathode may vary wide- 1y ,to meet the convenience and requirements of the application of the cell. .The container may be square, circular or of any other suitable design as to 'crosssection. The same is true of the anode, cathode and partition separating the for example, may be solid, grooved or made of screen. It may be cylindrical or ofany other suitable design; The same is true of the cathode.

largely dependent upon the surface of the electrodes and .the nearness of the electrodes to In general, the object of the design is to ob- .tain high currentand powerefliciency which is Very desirable results have been obtained in cells of the character illustrated in Figs. 1 and 2. In cells of this type good results have been obtained by placing the anode and. cathode. approximately equi-distant from the partition wall and preferably about one-half to about one and one-half inches therefrom.

Where it is desired to electrically insulate any portion of the apparatus, as for example the cathode, the insulating material should preferably be a substance which is not appreciably attacked by hydrofluoric 'acid and/or fluorine. In the construction of the apparatus, we have found that very desirable results are obtained in using as insulating materials inorganic solids which are -insoluble in anhydrous hydrofluoric acid, are not attacked by hydrofluoric acid or fluorine, and are not electrically conducting. As

specific examples of such insulating materialsmaterials, as for Very highly advantageous reobtained in the use of fused silver chloride as an insulating material andalso in the use of a mixture of aluminum fluoride and a wax such as ceresin. A specific composition of the latter type which may be mentioned consists of 30 parts of aluminum fluoride and 70 parts of ceresin. The method by which the inas such or in mixtures with each sulation is held in place may vary widely. In

insulating the cathode and wires of the electrolyte level indicating device, good results have been obtained by the methods described in connection with the drawings.

As indicated by the description with reference to the drawings, in operating the cell it is desirable, although not necessary, to provide control devices such as a device for-indicating the level of the electrolyte in the cell. It is further desirable and highly important for the continuous operation of the cell to add substantially anhydrous hydrofluoric acid to the electrolyte to replace that which has been vaporized and/or decomposed by the electrolysis. It will be recognized that the devices employed for these purposes may vary considerably. In the various devices for measuring the electrolyte level described in connection with the drawings, the

Example I An electrolyte consisting of I approximately by weight of potassium acid fluoride (KHFz) and 60% by weight of substantially anhydrous switch should prefhydrofluoric acid was introduced into a cell of the type described in Fig. 1, the liquid level being about that indicated in Fig. 1. The anode of the cell was constructed substantially of nickel. The container and cathode were constructed of iron, the cathode being electrically insulated from the top of the container by means of an insulating composition consisting of about 30 parts of aluminum fluoride and 70 parts of ceresin. The wires extending different depths into the well of the liquid level indicator were made of nickel and insulated from the well by the same material used in insulating the cathode. The liquid level indicator was operated on an alternating current. The partition between the anode and the cathode was constructed sub-- stantially of iron. That part of the partition below the level of the electrolyte and extending to the bottom of the cell was constructed of about mesh iron screen.. The cathode was constructed of about 8 mesh iron screen. The partition was equi-distant, about one-half inch, from the anode and cathode. A direct current was applied to the container and cathode corresponding to an anode current density of about 200 amperes per square foot. The temperature of the electrolyte wasrmaintained at about 30 C. to about 35 C. Fuorine was evolved at theanode and hydrogen at the cathode. The evolved gases were passed separately through condensers where the hydrofluoric acid was condensed, and continuously returned to the electrolyte; Fresh anhydrous hydrofluoric acid was added tothe electrolyte continuously in order to replace that which was decomposed by the electrolysis.

Example II The apparatus and process were the same as in Example I except that the electrolyte comprised substantially 30% potassium acid fluoride and 70% anhydrous hydrofluoric acid. The current density employed was about 300 amperes per square foot and the termperature of the cell was maintained at about 15 C.

Example III The apparatus and process were the same as described in Example I except that the cathode was madeof nickel instead of iron.

Example IV The process and apparatus were the same as described in Example I except that the diaphragm separating the anode and cathode was constructed substantially of cobalt instead of iron.

' Example V Example ,VI

The process and apparatus were the same as described in Example I except that the anode was constructed of an non-polarizing alloy comprising substantially nickel and iron.

Example VII In the process and apparatus described in Example I the metal container and the diaphragm separating the anode and cathode were rendered anodicalLv passive prior to, operation of the cell.

- Example l/III The process and apparatus were the same as' described in Example I except that fused silver chloride was used as an electrical insulating material instead of the aluminum fluoride-wax composition."

,The character of the electrolyte may vary 0 "within relatively wide limits. 'In general, the

electrolyte comprises hydrofluoric acid containing a material capable of imparting electrical conductivity thereto but which does not enter into the electrolytic action. It is preferable to employ hydrofluoric acid containinga soluble fluoride. The hydrofluoric acid should be of high purity and preferably substantially anhy- I drous. As specific examples of soluble fluorides which may be used may be mentioned rubidium fluoride, cobalt fluoride, potassium fluoride, po-

tassium acid fluoride, chromium fluoride, sodium fluoride and silver fluoride. Especially desirable results have been obtained in'the use of an electrolyte comprising substantially anhydrous hydrofluoric acid and an alkali metal fluoride, particularly potassium fluoride or potassium acid fluoride.

The composition of the electrolyte should preferably be such that it is fluid at the temperature of operation. In general, conductivity increases with increasedproportions of dissolved fluoride. The higher the proportion of dissolved fluoride, therefore, the greater the conductivity of the electrolyte and the better the eiflciency of the cell. 0n the other hand, a decrease in the proportion of fluoride decreases the conduc tivity of the electrolyte and the efliciency of the cell. In order to obtain the maximum conductivity at the temperature of operation without substantial precipitation of fluoride, it is preferable to employ an electrolyte composedof substantially anhydrous hydrofluoric acid containing suiflcient soluble fluoride in solution to be saturated at a temperature slightly lower than the temperature of operation.

The temperature may vary within relatively wide limits depending largely upon the composition of the electrolyte and the results desired.

The upper limit is fixed by the critical temperature (about 230 C.) of hydrofluoric acid. The lower temperature of operation is more or less determined by the solubility of the' fluoride, in the hydrofluoric acid, since it is desirable that the temperature be sufliciently high to prevent separation of the fluoride to such an extent as to make the electrolyte too thick for eflicient operation. For this purpose the temperature should preferably not be lower than about -20 C. Since hydrofluoric acid boils at about 19.5 C., relatively high temperatures tend to cause rapid volatilization of the hydrofluoric acid. Hence, in operating under atmospheric pressure where it is desired to recover the vaporized hydrofluoric acid, the higher the temperature the greater is the condenser capacity required. Generally speaking, therefore, it 'is' preferable to operate the cell at temperatures below about 100 C. and preferably within the'range of about 15 C. to about 0. Under these temperature conditions especially desirable results have been obtained in the use of electrolytes comprisat which the cell is operated issubject to considerable variation. In general, super-atmospheric pressures may be used to advantage in regulating the volatilization of the hydrofluoric acid from the electrolyte. By em ploying pressures greater than the partial pressure of the hydrofluoric acid in the electrolyte, the cooling capacity of condensers used to condense hydrofluoric acid vaporized from the electrolyte may be greatly reduced. Thus, the temperature of a condenser employed to condense hydrofluoric acid under atmospheric pressure (B. P. 19.5 C.) is lower than the temperature required for condensation under an absolute pressure of 20 pounds per square inch gauge (B. P. 47 C.) or pounds per square inch gauge (B. P. about 82. 0.). Condensers may be omitted from the construction of the apparatus, particularly in operating at relatively low temperatures or under super-atmospheric pressures; or, if desired, partial condensation'may be effected. In any case, it is desirable to replenish the electrolyte with anhydrous hydrofluoric acid in order to maintain it in a fluid condition. Super-atmospheric pressures may be obtained in any suitable manner, e. g., by closing or partly closing the cell outlets so that the total vapor pressure of the electrolyte at the temperature of operation is greater than atmospheric; or an outside source of pressure, as for exam 1e inert gas pressure, may be introduced.

The current density used in operating the cell may vary widely depending largely upon the production of fluorine desired. Other things being equal, a low current density will give a low production of fluorine, while a high current density will give a high production of fluorine. A high current density also tends to give a high heating effect so that it may be desirable to regulate the current density in accordance with the desired temperature of operation. Generally speaking, it is preferable to employ a current density'below about 300 amperes per square foot and preferably within the-range of about to 250 amperes per square foot.

The invention has the advantage that it provides a commercially practical process for the production of fluorine. Insofar as is known fluorine is not produced on a commercial scale at the present time. Previously proposed methods for the production of fluorine have been impractical for various reasons.- The so-called fused salt.

cells", that is, electrolytic cells, using as an electrolyte a fused fluoride, have the disadvantage that they operate at exceptionally high temperatures as, for example, to 400 C. Such cells have the further disadvantage that the electrolyte is made up and dried before using and when depleted of hydrogen fluoride is reworked. Ingen-,

eral, "fused salt fluorine cells may be defined as those in which the electrolyte is of such a composition that a metal fluoride will separate on cooling but hydrogenfluoride will not separate as such, whereas the electrolytes contemplated by the process of the present invention are those from which hydrogen'fluoride will eventually sep arate as such-on cooling. No anodic materials have heretofore been proposed which would withstand the action of hydrofluoric acid and fluorine to such an extent that they would be used commercially. As previously indicated, the anodic materials herein discussed are characterized by greaterresistance to the action of anhydrous hydrofluoric acid and fluorine than materials such as platinum and graphite. The extent to which various anodic materials will be'attacked by fluorine and hydrofluoric acid may vary widely, depending upon the composition of the electrolyte, the temperature, and other conditions of the electrolysis. It will be understood that even the anodic materials of the present invention may be attacked to some extentby hydrofluoric acid and/or fluorine, but the amount of metallic fluoride formed by such'attack is not substantial enough to prevent their use in commercial operations, whereas the contrary istrue in the case of materials such as platinum. To illustrate, under a given set of electrolyzing conditions using an electrolyte of the character herein described, an anodic material such as nickel may allow the recovery of.80% to 90% of the fluorine liberated by the electrolysis while platinum will allow the recovery of 0nly 20% to 30% of the liberated fluorine, the remainder reacting with the platinum and going into solution as a fluoride of platinum.

The type of cell construction described is considered to be broadly new not only for the electrolysis of hydrofluoric acid but for other electrolytic processes. Insofar as is known it has not been.- customary heretofore to construct a cell having-a metal container and a metal diaphragm between the anode and cathode, both the container and metal diaphragm being anodically.

passive. This design of a cell is particularly advantageous in that it eliminates insulation of the anode from surrounding parts and may cause a decided reduction in the amount of corrosion of the containerand metal diaphragm. A further advantage of the invention is the use of inexpensive materials for the construction of the various portions of the-apparatus. Other advantages are the provision of a means for the continuous replacement of hydrofluoric acid which has been 'volatilized or decomposed, the provision of a.

means for the circulation of the electrolyte to prevent concentration changes, the provision of suitable electrical insulating means, and the use of a multi-length electrical contact level indicator on a fluorine cell.

As many apparent and widely different embodiments of this invention may be made without departing from thespirit thereof, it is to be understood that we do not limit ourselves to the foregoing examples or description except as indicated in the following claims.

We claim: 7

1. In an electrolytic fluorine cell suitable for the electrolysis of anhydrous hydrofluoric acid, the construction of an apparatus including a nonpolarizing anode not substantially attacked by anhydrous hydrofluoric acid and fluorine, electainer for the electrolyte and a cathode electrically insulated from the anode and the metal container for the electrolyte.

2. In anelectrolytic fluorine'cell suitable for -trically connected to an anodically passive conthe electrolysis of anhydrous hydrofluoric acid,

the construction of an apparatus including an anode comprising substantially nickel, electrically connected to an 'anodicalLv passive container for the electrolyte and a cathode electri cally insulated from the anode and the metal container for the electrolyte.

3. In an'electrolytic fluorine cell suitable for the electrolysis of anhydrous hydrofluoric acid the combination of: an anodically passivifled metal container for the electrolyte; a non-polarizing anode not. substantially attacked by anhydrous hydrofluoric acid and fluorine inside of said container in contact with'the electrolyte; a

cathode not substantially attacked by anhydrous hydrofluoric acid and hydrogen inside of said container in contact with the electrolyte and larizing anode not substantially attacked by anhydrous hydrofluoric acid and fluorine inside of the container in contact with the electrolyte; a cathode not substantially attacked by anhydrous hydrofluoric acid and hydrogen inside of the container in contact with the electrolyte and electrically insulated from the anode; a means of withdrawing gases liberated at the anode and cathode without bringing them together; and a means for replenishing the electrolyte during the operation.

5. In an electrolytic fluorine cell suitable for the electrolysis of anhydrous hydrofluoric acid, the combination of: an anodically passivified base metal container for the electrolyte not substantially attacked by anhydrous hydrofluoric acid and fluorine; a non-polarizing metal anode not substantially attacked by anhydrous hydrofluoric acid and flurorine inside of said container in contact with the electrolyte; ametal cathode not substantially attacked by anhydrous hydrofluoric acid and hydrogen inside of said container in contact with the electrolyte and electrically insulated from the anode; a means of withdrawing gases liberated at the anode and cathode without bringing them together; and a means for replenishing the electrolyte during operation.

6. In an electrolytic fluorine cell suitable for fluoric acid and hydrogen inside of said container in contact with the electrolyte and electrically insulated from the container and from the anode; and a means of withdrawing gases liberated at the anode and cathode without bringing them together.

7. In an electrolytic fluorine cell suitable for the electrolysis of anhydrous hydrofluoric acid, the combination of: a metal container for the electrolyte constructed of a metal which is anodically passive and unreactive in anhydrous hydrofluoric acid; a non-polarizing anode not substantially attacked by anhydrous hydrofluoric acid and fluorine inside ,of said container incontact with the electrolyte and electrically con-- nected to the container; a metal cathodenot substantially attacked by anhydrous hydrofluoric acid and hydrogen inside of said container in contact with the electrolyte and electrically insulated from the container; an anodically passive metal partition inside of .the container between the anode and cathode out of contact therewith and joined with the container so as to divide it into an anode compartment and a cathode compartment, said partition being substantially gas-tight'above the electrolyte but sufficiently pervious in the electrolyte to allow passage thereof from one compartment toanother;

means for withdrawing gases from the anode and cathode compartments; and means for continuously replenishing the electrolyte.

8. The apparatus of claim 7, in which ode comprises substantially nickel.

9. The apparatus of claim 7, in which the anthe anode comprises substantially a non-polarizing a metal selected from the group consisting of iron, steel, cobalt, chromium, -magnesium, copper, alloys of any two or more of these metals and anodically passive alloys of one or moreoi these metals with nickel.

12. The apparatus of claim '7, in which the metal partition between the anode and cathode compartments is constructed substantially of a metal selected from the group consisting of iron, steel, cobalt, chromium and anodically passive alloys of these metals with each other or with nickel.

13. In an electrolytic fluorine cell suitable for the electrolysis of anhydrous hydrofluoric acid,

the combination of; a metal container for the electrolyte constructed substantially of iron; an anode constructed substantially of nickel situated inside of the container in contact with the electrolyte and electrically connected to the container; a cathode constructed substantially of iron situated inside of the container in contact Y with the electrolyte and electrically insulated from'the container; an iron partition. inside of the container between the anode and cathode out of contact therewith and joined to the metal container dividing it into an anode compartment and a cathode compartment, said partition being substantially gas-tight .above the electrolytebut sufliciently pervious in the electrolyte to allow passage thereof from one compartment to another; means for withdrawing gases from the anode and cathode compartments; and means for continuously replenishing the electrolyte.

14. In a relatively low temperature process for the production of fluorine, the step which comprises passing an electrolyzing current through a non-polarizing nickel-containing anode which is not substantially attacked by the action of hydrofluoric acid and fluorine in contact with an electrolyte composed of substantially anhydrous hydrofluoric acid containing suiflcient soluble fluoride in solution to be saturated at a temperature slightly lower than the temperature of operation, and maintaining a temperature of operation below about 100 C.

15. In a relatively low temperature process of producing fluorine, the step which comprises passing an electrolyzing current through an anode constructed substantially of nickel andin contact with an electrolyte composed of substantially anhydrous hydrofluoric acid containing suflicient soluble fluoride in solution to besaturated at a temperature slightly lower than the temperature of operation, and maintaining. the temperature of operation below about 100 C.

16. The process of ,claim 14 in which the electrolyte is composed of an unsaturated solution of a soluble fluoride in substantially "anhydrous hydrofluoric acid and the amount of hydroof the electrolyte.-

l7. In a process of producing fluorine, the step which comprises passing an electrolyzing current through an anode comprising substantially nickel and a cathode comprising substantiallyiron-in contact with an electrolyte composed substantially of about 30% to 50% potassium acid fluoride" and about 50%-to about-70%anhydrous hydro-, fluoric acid maintained at a temperature within the range of about C. to about 50C., continuously withdrawing fluorine liberatedat the, anode and hydrogen liberated at the cathode without bringing them together in the gas phase,'maintaining a current density below about 300 amperes per square foot and adding anhydrous hydrofluoric acid to the electrolyte during the operation in order to keep the electrolyte level substantiallyconstant.

18. The process of claim 17 in which the electrolysis is carried on under superatmospheric pressure.

19. In an electrolytic fluorine cell suitable for the production of fluorine, electrical insulation anodically passiviflediron.

means comprising substantially a material selected from the group consisting of non-conducting, inorganic compounds of metals having a specific gravity of at least 2.7 and which are substantially insoluble in anhydrous hydrofluoric acid and are not attacked byhydrofluoric acid or fluorine and mixtures of such solids with waxes. j

20. In an apparatus according to claim 19, fused silver chloride as an insulating means.

21. In an electrolytic fluorine-cell suitable for the electrolysis of anhydrous hydrofluoric acid, the construction of an apparatus including a metal container for the electrolyte which is anodically passivifled.

22. In an electrolytic fluorine cell suitable for the electrolysis of anhydrous hydrofluoric acid, the construction of an apparatus including a container for'the electrolyte which is constructed of 23; In an electrolytic fluorine cell for the production of fluorine by electrolysis of substantially anhydrous hydrofluoric acid containing. a soluble fluoride, the combination of: a metal container for the electrolyte constructed of a metal which is anodically passive, and unreactive in anhydrous hydrofluoric acid; a non-polarizing anode notsubstantially attacked by anhydrous hydrofluoric acid and fluorine inside of said container in contact with the electrolyte and electrically con-' nected to the container; a cathodenot substantially attacked by anhydrous hydrofluoric acid and hydrogen inside of said container in contact with the electrolyte; and an insulating means between the cathode and the container compris ing substantially a material selected from the group consisting 'of non-conducting inorganic compounds-of-metals having a specific gravity of at least 2.7 and which are substantially insoluble in anhydrous hydrofluoric acid and are not attacked by hydrofluoric acid and fluorine and mixtures with waxes; and anodically passive metal partition inside of the container between the anode and .the cathodeout of contact therewith and joined with the container so as to diaccuse fluoric acid corresponds toiat least 50% by weight ride into an anode compartment and a cathode compartmentisaid partition being substantially gas-tight above the electrolyte but sufllciently perviousiinthe electrolyte to allow passage thereofirom one compartment to another; and means for withdrawing ases from the anode and container for the electrolyte; a non-polarizing anode not substantially attacked by anhydrous hydrofluorlcacid and fluorine inside 01' the con-.

tainer in contact with'the electrolyte; a cathode not substantially attacked by anhydrous hydrofluoric acid and hydrogen inside of the container in contact with the electrolyte and electrically insulated from the anode; a means of withdrawing gases liberated at the anode and cathodewithout bringing them together; and a means including current conducting means disposed at indicating means, the current source in turn being connected to the container and the current conducting means disposed in the electrolyte,

whereby the indicating device registers when the "circuit is closed by contact of the current conducting means with the'electrolyte.

25. In an electrolytic fluorine cell suitable for for indicating the liquid level of the electrolyte diilerent depths within and above the electrolyte corresponding to the desired liquid level andelectrically insulated from the container, a current including an anodically passivifled metal container for the electrolyte, an anode and a cathode compartment in said container formed by a metal partition constructed of an anodically passivifled metal, said anode, partition andmetal container being electrically connected, and a cathode which is insulated from said other metal parts of the .cell and from the anode.

26. In an electrolytic fluorine cell suitable for the electrolysis of substantially anhydrous hydrofluoric acid containing a soluble fluoride, the construction ofan apparatus including a nonpolarizing nickel-containing anode, an iron container for the electrolyte, an anode and a cathode compartment in said container formed by an iron partition, said container and said partition .being electrically connected to the anode, and a cathode which is insulated from said other metal parts 0! the cell. v

27. In a process of producing fluorine, the step which comprises passing an electrolyzlng current through an anode comprising substantially nickel, and a suitable cathode in contact with an 

